You may have noticed that using a custom markup extension in a template sometimes lead to unexpected results… In this post I’ll explain what the problem is, and how to create a markup extensions that behaves correctly in a template.

The problem

Let’s take the example from the previous post : a markup extension which gives the state of network connectivity, and updates its target when the network is connected or disconnected :

If we disconnect from the network, we notice that the CheckBox is not automatically updated by the NetworkAvailableExtension, whereas it was working fine when we used it outside the template…

Explanation and solution

The markup expression is evaluated when it is encountered by the XAML parser : in that case, when the template is parsed. But at this time, the CheckBox control is not created yet, so the ProvideValue method can’t access it… When a markup extension is evaluated inside a template, the TargetObject is actually an instance of System.Windows.SharedDp, an internal WPF class.

For the markup extension to be able to access its target, it has to be evaluated when the template is applied : we need to defer its evaluation until this time. It’s actually pretty simple, we just need to return the markup extension itself from ProvideValue : this way, it will be evaluated again when the actual target control is created.

To check if the extension is evaluated for the template or for a “real” control, we just need to test whether the type of the TargetObject is System.Windows.SharedDp. So the code of the ProvideValue method becomes :

Now let’s run the application and unplug the network cable : the second CheckBox is updated, but the first one is not…

The reason for this is simple : there are two CheckBox controls, but only one instance of NetworkAvailableExtension, shared between all instances of the template. Now, NetworkAvailableExtension can only reference one target object, so only the last one for which ProvideValue has been called is kept…

So we need to keep track of not one target object, but a collection of target objects, which will all be update by the UpdateValue method. Here’s the final code of the UpdatableMarkupExtension base class :

The UpdatableMarkupExtension is now fully functional… until proved otherwise ;). This class makes a good starting point for any markup extension that needs to update its target, without having to worry about the low-level aspects of tracking and updating target objects.

A few months ago, I wrote a post where I explained how to automatically sort a GridView when a column header is clicked. I had mentioned a possible improvement : add a sort glyph in the column header to show which column is sorted. In today’s post, I present a new version of the GridViewSort class, which displays the sort glyph.

GridViewSort sample with sort glyph

To achieve this result, I used an Adorner : this is a component which allows to draw over existing UI elements, on an independant rendering layer.

The new version of the GridViewSort class can be used as before, in that case the grid displays default sort glyphs. These default glyphs are not particularly good-looking, so if you have some artistic skills you can provide you own images, as shown in the code below :